Regeneration of the injured central nervous system is difficult, but it has been reported in animal experiments that transplantation of embryo tissues, especially of neural stem cells, is effective. However, to obtain neural stem cells sufficient for therapy, many donations of aborted embryos are required. Moreover, since use of embryos is an ethical issue, practical clinical application of neural stem cells is difficult.
Thus, as a candidate of a transplantation material replacing neural stem cells directly isolated from embryos, neural stem cells that have been cultured and have proliferated in vitro have been a focus of attention. Neural stem cells are undifferentiated cells with self-replication ability and pluripotency. Since they proliferate unlimitedly by in vitro culture, they enable supply of a sufficient number of donor cells.
As a method for growing neural stem cells in vitro, the neurosphere method reported by Weiss et al. (Science 255, 1707-1710, 1992) is commonly used. Many examples of successful treatments have been reported, which were performed by transplanting neural stem cells proliferated through the neurosphere method to patients especially with intractable diseases, such as cerebral ischemia and neural degenerative diseases (Nature 422, 688-694, 2003).